1. Field of the Invention
This invention relates generally to storage, separation and analysis of ions according to mass-to-charge ratios of charged particles and charged particles derived from atoms, molecules, particles, sub-atomic particles and ions. More specifically, the present invention is a relatively small and portable device for performing mass spectrometry using a miniature toroidal configuration for a mass analyzer.
2. Description of Related Art
Mass spectrometry continues to be an important method for identifying and quantifying chemical elements and compounds in a wide variety of samples. High sensitivity and selectivity of mass spectrometry are especially useful in threat detection systems (e.g. chemical and biological agents, explosives) forensic investigations, environmental on-site monitoring, and illicit drug detection/identification applications, among many others. Thus, the need for a reliable mass analyzer that can perform in-situ makes a portable device even more relevant. Some key elements in developing portable mass spectrometers are reduction in size, weight and power consumption, along with reduced support requirements and cost.
Ion trap (IT) mass analyzers, by virtue of their simplicity, were selected by the inventors as candidates for miniaturization. For example, IT analyzers are inherently small, even as implemented commercially. IT analyzers have only a few ion optic elements, which do not require highly precise alignment relative to other types of mass analyzers. In addition, because they are trapping devices, multiple states of mass spectrometry (MS) can be performed in a single mass analyzer. The operating pressure for ion traps is higher than other forms of mass spectrometry allowing for less stringent pumping requirements. Furthermore, because the radio frequency (RF) trapping voltage is inversely proportional to the square of the analyzer radial dimension, a modest decrease in analyzer size results in a large reduction in operating voltage. This in turn results in lower power requirements. An added potential benefit of the reduced analyzer size is the shorter ion path length which may ease the vacuum requirements even further. As a practical matter, the shorter ion path length is especially important as some of the most limiting aspects of MS miniaturization are not in the ion optic components, but rather in the vacuum and other support assemblies.
The ability to miniaturize ion trap mass spectrometers hinges on several issues, including space charge and machining tolerance limits. Miniature ion traps exist today using conventional ion trap geometries (i.e. hyperbolic surfaces).
To understand the difference in relative size of the invention and state of the art devices, it should be understood that for this document, a conventional or full-scale toroidal and 3D mass analyzer, the radius of the toroidal trapping volume, also known as r0, is nominally considered to be approximately 1 cm.
It is also important that as these devices become smaller, the machining tolerances play an increasingly significant role in trapping field defects. Thus, it would be an advantage over the prior art to simplify the geometry to a design that is more easily machined.
Cylindrical ion trap mass analyzers have been miniaturized because the simplified, straight lines of a cylinder are considerably easier to machine than hyperbolic surfaces, especially in small dimensions. When the geometry of the analyzer electrodes deviates significantly from the theoretical geometry, as is the case for cylindrical ion traps, corrections are needed to restore the trapping field potentials to their theoretical values. Modeling and simulation programs have been used extensively in this undertaking.
Disadvantageously, the gains from reducing analyzer size (e.g. increased portability due to lower weight and smaller size, lower RF generator power, and relaxed vacuum requirements) are understandably offset by a reduction in ion storage capacity in state of the art mass analyzers. Concomitant with this reduced capacity is an earlier onset of space charge conditions, based on ion-ion repulsion, which results in reduced mass resolution and mass peak shifts. Efforts to address this constraint in ion mass spectrometers have lead to several different approaches. For example, arraying several reduced volume cylindrical ion traps is one approach to recovering the lost ion capacity. More recently, linear ion traps with either radial or axial ejection have also been developed. The increased ion storage capacity is due to the volume available throughout the length of the two-dimensional quadrupole rod array. These devices are now readily available in commercial versions.
For reasons similar to those where cylindrical ion trap geometries are used to approximate the 3D quadrupolar ion trapping field, a rectilinear ion trap has been reported that uses a rectangular rod assembly instead of the more conventional hyperbolic quadrupole rod surface. All of these linear devices provide an increase in ion storage capacity by employing a traditional 2D quadrupole with ion gates on either end of the quadrupole array. Arrays of linear quadrupoles have also been reported.